The size of photoresist features vary with respect to photoresist thickness for a particular wafer substrate. Therefore, it is important to determine a photoresist thickness that will provide the desired photoresist feature size. Conventional processes for determining a thickness that will provide a desired photoresist feature size require the generation of a swing curve.
A swing curve is typically generated by preparing a dozen or more identical semiconductor wafers. The semiconductor wafers can be bare wafers or can have other layers and/or structures formed thereover. The layers and/or structures formed over the semiconductor wafer typically include those layers and/or structures that may affect photoresist feature size in the fabrication process. For example, when a photoresist layer is to be formed over a metal layer, the metal layer is deposited over each of the dozen or more semiconductor wafers.
The dozen or more semiconductor wafers are then placed in a photoresist coat track. Each wafer is then coated with the type of photoresist that is to be used in the semiconductor fabrication process. The track recipe for each wafer is different so as to coat each wafer with a different thickness of photoresist. The thickness of the photoresist is then measured on each of the dozen or more twelve semiconductor wafers.
All twelve semiconductor wafers are then exposed and developed so as to produce an identical photoresist feature on each of the dozen or more wafers. For each semiconductor wafer, the size of the photoresist feature is measured. The size of the photoresist feature is plotted relative to the photoresist thickness. Typically, thickness of photoresist is plotted on the x-axis and size of resist feature is plotted on the y-axis. A curve (swing curve) is then generated that fits the plotted points.
The process of generating a swing curve is typically expensive and time consuming due to the number of test wafers that must be fabricated and measured. Also, random or systematic process variation between wafers can result in the generation of a swing curve that is not accurate. When the swing curve is not accurate, costly fabrication defects occur, resulting in reduced yield and potentially resulting in device failure.
Thus, there is a need for a method for generating an accurate swing curve. In addition, there is a need for a method for forming a photoresist feature having a desired size. Moreover, there is a need for a method for determining photoresist thickness that can be used in a semiconductor fabrication process. The present invention meets the above needs.